Re: [FFmpeg-devel] [PATCH 5/5] avformat/pcm: Use 64bit in bitrate computation

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Marton Balint:
> 
> 
> On Thu, 4 Apr 2024, Michael Niedermayer wrote:
> 
>> Fixes: signed integer overflow: 65792 * 65312 cannot be represented in
>> type 'int'
>> Fixes:
>> 67819/clusterfuzz-testcase-minimized-ffmpeg_dem_WADY_fuzzer-5236100912185344
>>
>> Found-by: continuous fuzzing process
>> https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
>> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
>> ---
>> libavformat/pcm.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/libavformat/pcm.c b/libavformat/pcm.c
>> index 051e86dd464..a774dbc3726 100644
>> --- a/libavformat/pcm.c
>> +++ b/libavformat/pcm.c
>> @@ -41,7 +41,7 @@ int ff_pcm_default_packet_size(AVCodecParameters *par)
>>     /* Don't trust the codecpar bitrate if we can calculate it
>> ourselves */
>>     if (bits_per_sample > 0 && par->sample_rate > 0 &&
>> par->ch_layout.nb_channels > 0)
>>         if ((int64_t)par->sample_rate * par->ch_layout.nb_channels <
>> INT64_MAX / bits_per_sample)
>> -            bitrate = bits_per_sample * par->sample_rate *
>> par->ch_layout.nb_channels;
>> +            bitrate = bits_per_sample * (int64_t)par->sample_rate *
>> par->ch_layout.nb_channels;
> 
> LGTM, thanks.
> 
> I wonder why we usually cast the second operand and not the first to 64
> bit, since cast has higher precedence than multiplication, it should not
> matter, should it?
> 

Presuming that all variables here have integer conversion rank <=
int64_t (true here for normal systems), then it does not matter: Casting
the first operand would automatically promote all the other operands to
int64_t, too; but if you add the cast to the last operand only, the
first multiplication will be performed with ints only.

- Andreas

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